THE BEGINNINGS OF LIFE. 207 



first rudiments of the egg 1 in these cases. Certain it 

 is, however, as he and almost all other embryologists 

 admit that, even in the higher animals, the yolk is 

 always formed by a mere aggregation of granules and 

 of a mucilaginous substance, subsequently becoming 

 limited by a vitelline membrane. And yet the granular 

 substance of the yolk constitutes, by its segmentation, 

 the initial embryonic mass of the future animal. In 

 certain animals, indeed, the yolk mass is apparently all 

 that exists : the germinal vesicle seems to be absent. 



Seeing the undecisive nature of the evidence as to 

 the precise mode of origin of the c germinal vesicle,' it 

 is desirable to learn whether its subsequent fate bears 

 out the generally prevalent notion of its immense im- 

 portance as a constituent of the ovum. What follows 

 refers equally to ova produced by either of the two 

 methods above referred to to those which have a free 

 origin within tubular organs, or to those arising in the 

 midst of a more or less solid organ. 



Before the mingling of the contents of the sperm- 

 cells with the granules of the vitelline substance that 

 is before fecundation 2 has taken place it seems to be 



1 The ' clear point ' which next makes its appearance, the rudiment of 

 the future germinal vesicle, may be evolved as a gradually increasing 

 dot of homogeneous mucilage after the same fashion as the nucleus is 

 now known to appear in so many cells which are in process of evolution. 



2 It may be well to quote here some philosophical remarks of Dr. 

 Allen Thomson bearing upon the phenomena of fecundation. He says : 

 ' The physiologist agrees, for the sake of convenience of expression, to 

 adopt the terms power, property, force, &c., to denote the conditions 

 necessary for the occurrence of certain actions or changes 



